Particle-beam device equipped with magnetic means for transversely displacing parts thereof

ABSTRACT

A particle-beam device has a beam axis and a housing surrounding the axis wherein a moveable structure is disposed. An apparatus for moving the structure transverse to the beam axis has first magnet pairs disposed at opposite sides of the structure and positioned so that their respective magnetic forces act in a direction transverse to the beam axis. The two magnets of each pair of the first magnet pairs are mutually adjacent, one of the two adjacent magnets being arranged on the outer periphery of the structure and the other of two the adjacent magnets being fixedly mounted with respect to the housing. Second magnet pairs are positioned relative to the beam axis so that their respective magnetic forces act in a direction parallel to the beam axis, the magnets of each pair of the second magnet pairs being mutually adjacent. One magnet of each pair of the second magnet pairs is arranged on a surface of the structure, the surface being transverse to the beam axis. The other magnet of each pair of the second magnet pairs is fixedly mounted with respect to the housing.

United States Patent Braun et al.

[151 3,678,270 14 1 July 18,1972

[72] inventors: Dieter Braun, Berlin, Germany; Klara Heinemann, Palo Alto, Calif.

[73] Assignee: Siemens Aktiengesellsehait, Berlin, Germany [22] Filed Sept. 8, 1970 [21] Appl.No.: 70,014

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church Att0rney-Curt M. Avery, Arthur E. Wilfond, Herbert L. Lerner and Daniel J.' Tick ABSTRACT A particle-beam device has a beam axis and a housing surrounding the axis wherein a moveable structure is disposed. An apparatus for moving the structure transverse to the beam axis has first magnet pairs disposed at opposite sides of the structure and positioned so that their respective magnetic forces act in a direction transverse to the beam axis. The two magnets of each pair of the first magnet pairs are mutually adjacent, one of the two adjacent magnets being arranged on the outer periphery of the structure and the other of two the adjacent magnets being fixedly mounted with respect to the housing. Second magnet pairs are positioned relative to the beam axis so that their respective magnetic forces act in a direction parallel to the beam axis, the magnets of each pair of the second magnet pairs being mutually adjacent. One magnet of each pair of the second magnet pairs is arranged on a surface of the structure, the surface being transverse to the beam axis. The other magnet of each pair of the second magnet pairs is fixedly mounted with respect to the housing.

21 Claims, 7 Drawing Figures SHEET 2 BF 4 Fig. 3

11 3 CONVER TOR CON VE R TOR [j] MEMORY V l 110 V PATENTEU JHLI 8 1972 SHEET 3 [1F 4 Fig. 5

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126a i Fig 6 1 1 L22 :IL J

51 52 MEMORY 3 1 129 130 131 132 DETECTOR CONVERTUR REGULATUR PARTICLE-BEAM DEVICE EQUIPPED WITHMAGNETIC MEANS FOR TRANS VERSELY DISPLACING PARTS THEREDF Our invention relates to particle-beam devices equipped with apparatus for displacing parts or structure transversely with respect to the beam axis. More particularly, our invention relates to particle-beam devices equipped with magnetic means for transversely displacing structures. Our invention also relates to excitation current circuits thatfunction in cooperative relation with the magnetic means.

With the imaging of specimen details in modern particlebeam devices such as electron microscopes, ion microscopes or deflection apparatus, there is obtained a resolution in order of magnitude of a few A. The requirement for ever better. resolution in the image places ever higher requirements on the stability of the device, whereby not only the stability of the beam voltage and the lens current is affected, but also the mechanical stability. In this connection, an especially critical point is the stability of the position of the specimen under investigation. It is known that movements ofthe specimen occur often during the microscopic examination of a specimen which render photographic pictures unusable. If it is desired to achieve a resolution of very few A, during the total time of illumination of photographic material, if necessary for many minutes, the specimen drift can amount to only fractions of an A.

The undesired specimen movements have been attributed to temperature changes and thereby shown that through the working of the particle-beam, a heating of the specimen occurs which produces a disturbance of the temperature balance between different parts of the particle-beam device important for the specimen position. Or, the heating of the specimen has the effect that the heat balance required for the specimen stability adjusts itself only after a long period of time because of the heat transfer resistance occasioned by the configuration of the parts of the particle-beam device.

New investigations by, Heide have shown. thermal influences are not the only cause of disturbing specimen wanderings. In this connection. reference may be made to the Veroffentlichungen der 4. Europiiischen Konferenz iiber Electromikroskopie," Rome I968, pages2l2 and 2l3. Heide could establish that a disturbing specimen drift with consideration of thermal conditionscould only be precluded with certainty if the driving mechanism for displacing the specimen table in a plane transverse to the beam axis would be uncoupled after completion of the displacement movement. However,'there are still mechanical forces which work on the table in individual directions which are very small compared to the actual displacement forces required and applied by the drive mechanism, but which nonetheless have the result that the specimen displaceable table effects so-called micro-movements in the region of its glade faces of the table during microscopic investigations. 7

In this above-mentioned disclosure, Heide teaches a meachanical solution to this problem which providesessentially for a two part specimen displaceable table whose parts work coupled together only during displacement movements. The actual displaceable table is supported with glide feet on a surface of the objective in the usual manner and is held in contact with this surface by means of springs. In addition aringlike second table part is provided which is positioned against outwardly projecting flanges of the actual table by means of springs acting from below and thering-like table functions to reduce the pressure of the table against the objective. A mechanical arm serves to ensure that the two parts of the table are disengaged except during the time of the displacement movement. During the displacement movement, the outer table portion lifts the actual table somewhat and the driving means standing continuously in mechanical connection with the ring-like portion can function by means of the coupling so established, on the actual table provided with a specimen. There is taught by US. Pat. No. 2,496,051, a specimen displacement apparatus which does not take into account the Heidesche recognition of the significance of the uncoupling of the drive means. In this apparatus, a displaceable specimen table is similarly disposed on the upper surface of the objective lens that is constructed to provide a friction surface and can bedisplaced by magnetostrictive means via coils arranged to the side of the table. These coils are excited with a sawtooth voltage. The table makes small jumps in specific directions in dependence upon the amplitude and the edge slope of the saw-tooth pulses. As soon as the saw-tooth voltage is disconnected, the table sustains no further tangential forces.

This known arrangement with its practical construction nonetheless provides considerable difficulties. These difficulties are introduced because the objective surface serving as a friction surface must be. constructed so that on the one hand, it does not hinder the desired movement of the table, that is, have the smallest possible friction, and, on the other hand, provide friction after the coils constituting the displacing mechanism are disconnected, that is, the friction must be large enough to prevent with certainty unwanted specimen movements by means of displacements from the outside. In addition, the known arrangements appear to provide no means that ensure that the specimen table will be directed in the desired direction notwithstanding the complicated friction relationships in the movement of the'table on the supporting surface.

It is an object of our invention to provide an apparatus for moving parts in a particle-beam' device.

It is another object of our invention to provide an apparatus for displacing a structure'or parts in a particle-beam device which overcomes the disadvantages of the known solutions.

More particularly, it is an object of our invention to provide an apparatus for moving a structure or parts located in the evacuated portion of a particle beam device with accuracy in a desired direction to a desired position. Subsidiary to this object we provide such apparatus which will ensure that the part once positioned will not become dislocated by unavoidable mechanical movements or by temperature unbalance between parts of the particle-beam, device or the like.

According to the'invention, a particle-beam device, especially an electron microscope, is equipped with magnet means to move parts located in the evacuated part of the device transverse to the beam axis. The pats to be moved may consist of the specimen table or they maybe diaphragms or phase rotating plates or foils that have to be centered. Such phase rotating plates often have a definite pattern for their phase rotating operation when viewed with respect to their crosssections, so that they have to be precisely centered with respect to the beam. This also applies to special diaphragms such as the Hoppe zone diaphragm with alternating ring-like beam blocking and beam passing zones...

According to a feature of the invention, we provide an apparatus for displacing a structure within a particle-beam device which comprises first magnet pairs disposed at opposite sides of the structure so that their force effect is directed transverse to the beam axis. Each pair consists of two mutually adjacent magnets. One, magnet of each of these first magnet pairs is arranged, on the outer periphery of the structure whereas the other magnet of each pair is fixedly mounted on the particle-beam device. The apparatus also comprisesv second magnet pairs, each of said second pairs having two mutually adjacent magnets disposed so that their force effect is directed parallel to the beam axis. One magnet of each of these second pairs is arranged on a surface of the structure transverse to the beam axis, whereas the other magnet of each pair is fixedly mounted on the particle-beam device. At-least one magnet of each pair of the first and second magnet pairs is an electromagnet. The excitation of these first and second pairs is adjusted so that only during the. displacement of thestructure in the desired direction do the first magnet pairs lying opposite each other apply forces to the structure in this direction. The second magnet pairs ensure a low friction contact of the structure on a fixed surface disposed transverse to the beam axis during the displacement movements. After the movement of the structure has been completed, the excitation of the first magnet pairs is disconnected. When the excitation of the electromagnets of the first magnet pairs is disconnected, the second magnet pairs provide for a friction force between the structure and a fixed load or support surface thereby precluding a transverse movement of the structure.

The invention makes unnecessary the use of control members passing vacuum tight into the microscope column for adjusting the position of a part or structure such as a specimen table or diaphragm. According to the invention, separate magnets are provided for displacing a structure by means of magnetic force, a portion of these magnets produce forces in a direction perpendicular to the beam axis for obtaining movements of the structure transverse to the beam axis, whereas the other magnets serve to establish friction forces at optimum values between the structure and a loading surface, these values corresponding to appropriate steps in the structure displacement procedure. The following description of the preferred embodiments of the invention will show that this division of the magnetic means affords considerable advantages for the actual movement of the part and for the adjustment of the friction forces notwithstanding the use of similar magnetic structure.

An adjusting apparatus for a specimen table is known, for example, from German Pat. No. 1,078,703 wherein the table is moveable transverse to the beam axis via mechanical drives. Differences'in pressure force on the loading surface occurring during displacement movements and during the performance of microscope investigations are adjustable with the aid of pneumatic means.

German Printed Pat. Application No. 1,300,992 describes a particle-beam device with parts moveable on a load surface transverse to the beam axis. The device is similarly equipped with mechanical adjusting means with which changes in loading pressure forces are achieved by pneumatic or hydraulic means in combination with springs at the time of the transition from shifting movements to operation ofthe microscope.

These known arrangements are concerned with mechanical adjusting means which do not consider the Heidesche require,- ment after uncoupling the part or structure from the drive mechanism after positioning the structure.

Accordingly, it is another object of our invention to providean apparatus for displacing a structure transverse to the beam axis which precludes any unwanted displacement of the structure from the desired location when the means for transversely displacing the structure is disconnected.

According to a feature of the invention, the excitation of the second magnet pairs is adjustable so that the structure to be displaced is in contact with a loading surface so that the friction force therebetween is small during displacement of the structure and greater when the excitation of the electromagnets of the first, magnet pairs is disconnected.

For the arrangement of the loading surface with regard to the structure to be displaced there are two variants, the first, is to apply a suitable excitation to the second magnet pairs so that the structure applies a force to the loading surface arranged above the structure in accordance with the corresponding phase of the positioning work cycle, the force being a pulling or a pressure force depending upon whether the second magnet pairs are disposed above or under structure. It can be preferable, to locate loading surface beneath the structure to be displaced and so take advantage of its weight.

Up to the present time, only two conditions on the loading surface for the displaceable structure were considered, namely, the achievement of a relatively small pressure force during the structure displacement and a larger pressure force to preclude unwanted movements when the excitation of the first magnetic pairs is turned off. Accordingly, it is another feature of our invention to arrange the structure with clearance between an. upper and a lower surface so that it lies on the upper surface while the structure is displaced and during the time that the excitation of the electromagnets of the first magnet pairs is disconnected, and thereafter, as consequence of another change in the excitation of the electromagnets of the second magnet pairs, he structure drops to the lower surface.

During the displacement of the structure and the turn-off of the excition of the first magnet pairs, with these variants of the invention, there results a pressing of the structure against the upper loading surface. For cases where an especially certain position is required, such as in the case of the position of a diaphragm, a phase displacement plate or the like, there exists the possibility of permitting the structure to position itself upon the lower loading or support surface by using the weight of the structure and by increasing the friction force between the structure and the supporting surface by increasing the excitation in the electromagnets of the second magnet pairs. In some cases, especially with heavy parts, it is sufficient for this purpose to simply disconnect the excitation of the second magnet pairs, so that the structure falls from its position on the upper loading surface by means of gravitational force. The clearance between the structure to be positioned and the two supporting or loading surfaces is held as smallas possible. In other cases, with the transition into this operating position, the polarity of the electromagnets of the second magnet pair are reversed. I

One magnet of each pair of the second magnet pairs can be fixedly mounted to the particle-beam device whereas the other magnet of each pair of the second magnet pair can be positioned above the one magnet so that their pole faces are mutually adjacent. in this manner, the pole faces of the magnets of the second magnet pairs mounted to the device can constitute one of the two supporting surfaces.

To preclude statistical uncertainty and obtain good mechanical stability, three second magnet pairs are provided with the electromagnet of each pair being excited with the same number of ampere turns. In all cases, these three magnet pairs are arranged in rotational symmetry with respect to the beam axis. 7

German Printed Pat. Application No. 1,300,992 describes a particle-beam device with parts moveable on a load surface transverse to the beam axis. The device is similarly equipped with mechanical adjusting means with which changes in loading pressure forces are achieved by pneumatic or hydraulic means in combination with springs at the time of the transition from shifting movements to operation of the microscope.

These known arrangements are concerned with mechanical adjusting means which do not consider the Heidesche requirement after uncoupling the part or structure from the drive mechanism after positioning the structure.

Accordingly, it is another object of our invention to provide an apparatus for displacing a structure transverse to the beam axis which precludes any unwanted displacement of the structure from the desired location when the means for transversely displacing the structure is disconnected.

According to a feature of the invention, the excitation of the second magnet pairs is adjustable so that-the structure to be displaced is in contact with a loading surface so that the friction force therebetween is small during displacement of the structure and greater when the excitation of the electromagnets of the first magnet pairs is disconnected.

For the arrangement of the loading surface with regard to the structure to be displaced there are two variants, the first, is to apply a suitable excitation to the second magnet pairs so that the structure applies a force to the loading surface arranged above the structure in accordance with the corresponding phase of the positioning work cycle, the force being a pulling or a pressure force depending upon whether the second magnet pairs are disposed above or under the structure. It can be preferable, to locate loading surface beneath the structure to be displaced and so take advantage of its weight.

Up to the present time, only two conditions on the loading surface for the displaceable structure were considered, namely, the achievement of a relatively small pressure force during the structure displacement and a larger pressure force to preclude unwanted movements when the excitation of the first magnetic pairs is turned off. Accordingly, it is another feature of our invention to arrange the structure with clearance between an upper and a lower surface so that it lies on the upper surface while the structure is displaced and during the time that the excitation of the electromagnets of the first magnet pairs is disconnected, and thereafter, as consequence of another change in the excitation of the electromagnets of the second magnet pairs, the structure drops to the lower surface.

During the displacement of the structure and the turn-off of the excitation of the first magnet pairs, with these variants of the invention, there results'a pressingof the structure against the upper loading surface. For cases where an especially certain position is required, such as in the case of the position of a diaphragm, a phase displacement plate or the like, there exists the possibility of permitting the structure to position itself upon the lower loading or support surface by using the weight of the structure and by increasing the friction force between the structure and the supporting surface by increasing the excitation in theelectromagnets of the second magnet pairs. In some cases, especially with heavy parts, it is sufficient for this purpose to simply disconnect the excitation of the second magnet pairs, so that the structure fallsfrom its position on the upper loading surface by means of gravitational force. The clearance between the structure to be positioned and the two supporting or loading surfaces is held as small as possible. In other cases, with the transition into this operating position, the polarity of the electromagnets of the second magnet pair are reversed, he one on topof the other.

One magnet of each pair of the second magnet pairs canbe fixedly mounted to the particle-beam device whereas the other magnet of each pair of the second magnet pair canbe positioned above the one magnet so that their pole faces are mutually adjacent. In this manner, the pole faces of the magnets of the second magnet pairs mounted to the device can constitute one of the two supporting surfaces.

To preclude statistical uncertainty and obtain I good mechanical stability, three second magnet pairs are provided with the electromagnet of each pair being excited with the same number of ampere turns. In all cases, these three magnet pairs are arranged in rotational symmetry with respect to the beam axis.

An arrangement of the magnet pairs with respect to the displaceable structure will be described with reference to FIGS. 1 and 2 of the drawing. An excitation control circuit that functions in cooperative relation with the magnet pairs will follow the descriptive passages relating toFIGS. l and 2.

FIG. 1 is a'sectional view of the pertinent portion of the column of an electron microscope whereat the magnetic means according to the invention is located.

FIG. 2 is a schematic diagram of an arrangement of magnet pairs for adjusting the position of a specimen table.

Although the description relates to the special case of transversely positioning a specimen table, this should not be considered a limitation as to the scope of the invention. The invention is readily applicable for positioning other parts in a particle-beam device. Referring to FIG. 1, reference numeral 1 designates the vacuum wall of the vacuum space of an electron microscope. The vacuum wall I is concentric with the beam axis 2, so that this axis also is the axis of the microscope.

The specimen table 5 is located with clearance between the two disc-shapedparts 3 and 4 fixedly secured to the wall 1 and having openings surrounding the particle beam. The table 5 has a central opening 6 for accommodating a specimen cartridge during microscope operation.

In the illustrated operating position, forces are applied to the table 5 by means of magnet pairs 7, 8 and a third magnet pair of same type not shown in FIG. 1 in an upward direction so that the slide feet 9 and I0 areagainst the loading surface 11 of part 3. Also, two magnet pairs 12 and 13 are disposed on diametrically opposite sides of the beam axis. Not shown in FIG. 1 are two additional magnet pairs positioned perpendicular of magnet pairs 12 and 13; these additional pairs act to dis place the table 5 in a direction perpendicular to the plane of the drawing.

Magnet pairs 12 and I3 belong to what will be referred to as the first magnet pairs and each pair consists of two electromagnets, namely I4, 15 and 16, 17 respectively. The electromagnets 14, I5 and the electromagnets l6, 17 form with their adjacent pole faces, respective air gaps I8, 19. The electromagnets l5 and I6 are secured to the outer surrounding surface of the table 5, whereas the outer magnets 14 and [7 are fixedly mounted as indicated. The excitation of the magnets of the two magnet pairs is adjusted so that the magnets of each pair I4 and 15 or 16 and I7, oppose each other. With different values of excitation for the individual magnet pairs, it is possible to move the specimen in the table perpendicular to the beam insofar as the friction conditions between the slide feet 9 andl0 and the load surface 11 will permit.

The excitations of electromagnets 20 and 21 comprising the pair 7 or the electromagnets 22 and 23 comprising pair 8 are adjusted in the same way. I-Iere also, the excitations are selected for obtaining a repelling force, especially in view of the weight of the table 5 provided with various magnets, so that the glide surface 11 serves as an effective guide for the table anddoes not act to effect a braking action when the table is displaced.

The magnet pairs 7 and 8 as well as the third magnet pair not discernible in FIG. I are all disposed beneath the table 5 and are collectively designated as the second magnet pairs.

FIG. 2 illustrates the arrangement of the first magnet pairs which comprises magnet pairs 32 and 33 in addition to magnet pairs 12 and 13. The first magnet pairs are shown arranged with respect to displaceable table 5. The magnetic pairs are put together from electromagnets often, however, the magnets secured to the part 5 or built directly into the part are permanent magnets.

FIG. 2 shows two magnet pairs 35 and 36 which are designated as the third magnet'pairs and are likewise arranged with respect to the table 5 and are disposed on diametrically opposite sides of the beam axis 2. The magnet pairs 35 and 36 form an angle of 45 with the respective magnet pairs I2, 13, 32 and 33 of the first magnet pair group and serve to guide table 5 while the movement of the latter transverse to the beam 2 is effected by appropriately exciting the magnets of the first magnet pairs. The third magnet pairs comprising magnet pairs 35 and 36, which in'turn comprise electromagnets 38, 39 and 40, 41 respectively, operate to prevent a turning of the table 5 while the desired displacement thereof is effected by means of such a pole orientation and excitation of the electromagnets that pulling forces develop between the magnets 38, 39 or 40, 41. However, the forces applied to table 5 are held smaller than the forces applied by the first magnet pair group, for example, by the appropriately selected excitations of the magnets of the third magnet pair, so that the pulling force produced by one of these magnet pairs is not too great when the forces between the magnet pairs 35 and 36 of the third magnet pair group become unbalanced. This weakening of the magnetic forces can be obtained, for example, by dimensioning the gaps between the magnets comprising the third magnet pairs such that these gaps are larger than the corresponding gaps of first magnet pairs.

In carrying out a displacement of the table 5, the following procedure can be used: First, the magnets of the second magnet pairs 7 and 8 are excited in the already described manner, so that they lightly press table 5 against the upper surface 11; then, the first magnetic pairs I2, 13, 32 and 33 are suitably energized for carrying out the actual displacement movement. Before deenergizing the first magnet pairs 12, I3, 32 and 33, the applied force of the two magnet pairs 7 and 8 is considerably increased, so that no undesired movement of the table can occur while the first magnet pairs 12, 13, 32 and 33 are deenergized. Subsequently, the two magnet pairs 7 and 8 are turned off, so that the table 5 falls through the very small distance of the air gaps 50 and 51 between the pole faces of the magnets comprising the pairs 7 and 8. Because of the table weight, there is a sufficiently high coefficient of friction in the region of the contacting pole faces of the magnets of pairs 7 and 8.

It is also possible to reverse the polarity of the second pair so that the weight of table is made more effective with regard to increased friction action by means of magnetic forces; this is especially applicable when the part being positioned is a diaphragm or the like.

In the following, suitable values of the excitation of the electromagnets of the various magnet pairs as well as applicable embodiments of the excitation current circuit are described.

During the displacement movements, the electromagnets of the opposite lying first magnet pairs, that is, the magnet pairs performing transverse displacements, can be excited with such polarity and intensity, that the magnets of the first magnet pair toward which motion is directed exert a larger repelling force on each other than do the magnets of the first magnet pair away from which motion is directed. As compared to the use of pulling forces, the repelling forces offer the advantage that undesired movements of the part in the pertinent direction are opposed. Switching technology permits attaining this effect in the manner that for obtaining a simultaneous adjustment of the excitation of the electromagnets of respective first magnet pairs lying opposite each other, a common potentiometer is provided in a supply current circuit, so that when the voltage in one electromagnet is increased, it is decreased by the same in the other electromagnet.

The different excitations of the electromagnets of the second magnet pairs are adjusted by means of additional potentiometers. Also provided are throw-over switches.

Preferably, a sequence switch is provided, which when actuated first closes the excitation circuit for the second magnet pair, so that the displaceable part is brought into the position best suited with regard to the friction conditions. The sequence then closes the excitation circuit for the first magnet pairs. One advantage of the invention is that an automatiza tion of the displacement procedure is achieved with the simplest of means, whereby failure conditions are eliminated because of the enforced control of the individual movement steps and their corresponding modes of operation.

It can be required to provide additional means to secure the displaceable part against undesired turning motions when the part undergoes its linear displacement movement. According to a preferred embodiment of the invention, in this connection, third magnet pairs are provided to ensure against rotation movements. Each magnet pair of the third magnet pairs has one magnet arranged on the outer periphery of the part and the other magnet fixedly mounted on the particle-beam device. Preferably, the third magnet pairs form an angle of 45 with respect to the first magnet pairs.

Whereas, for transverse displacements, the electromagnets of first magnet pairs should be so poled, that they subject the displaceable part to repelling forces, it is advantageous with regard to the third magnet pairs, to permit he third magnet pairs to act with pulling forces of equal magnitude, the third magnet pairs lying opposite each other with respect to the displaceable part. In this connection, the forces of the third magnet pairs acting on the displaceable part are held smaller than the forces of the first magnet pairs acting on the part, for example, by providing larger air gaps between the magnets constituting respective pairs, so that the means for preventing rotation formed in this manner exercises no pulling forces on the part in the sense of a transverse displacement of the part.

lt is not necessary that every magnet be an electromagnet. [t is sufficient, forchanging the acting forces of the magnet pairs if one magnet of each pair is an electromagnet. To make it unnecessary to provide current to the displaceable part, the magnets affixed thereto are permanent magnets, whereas the magnets fixed to the particle-beam device are electrically excited.

In FIG. 3 is shown a schematic diagram of a switching circuit for adjusting the position of a microscope part such as the specimen table of FIG. 2.

Referring to FIG. 3, the three electromagnets belonging to the second magnet pairs are designated by 21, 23 and 25, two of the second magnet pairs 7, 8 being shown in FIG. 1. 1n the right current branch are coils l4 and 17 arranged so as to provide oppositely directed magnetic flux lines. Coils 14 and 17 belong respectively to two opposite lying magnet pairs 12 and 13 of the first magnet pairs. The following will relate, for example, to the first magnet pairs 12 and 13 for displacing the table in x direction.

A direct-current voltage source U is connectable by a special switch to a variable resistor 65 on the one hand and to a potentiometer 66 and the coils of the various magnetic pairs on the other hand. Variable resistor 65 serves to adjust the common excitation in the three coils 21, 23 and 25. The potentiometer 66 functions to reduce the current through one of the coils 14 and 17 in the same amount as it serves to increase the current in the other of the coils 14 and 17.

An essential component of the switching circuit is the switch 67 which is constructed so that it automatically introduces the individual modes of the displacement cycle when it is actuated. The switch 67 has two mechanically joined switching arms 68 and 69 which are electrically isolated with respect to each other and permits a connection of the voltage source U to the current circuit on the left as well as a connection of the source to the current circuit on the right. The illustrated position of the switch is the position thereof at which switching arm 68 connects the full value of potentiometer 70 into the excitation circuit 65-21-23-25 of the second magnet pairs, so that practically no current flows therein, whereas, the switching arm 69 makes no connection between the voltage source U and the coils 14 and 17 of the first magnet pairs 12, 13.

Before a current can flow in the right current circuit and a corresponding displacement of the table undertaken, the part to be moved must be pressed gently against the upper support surface 11 by means of the second magnet pairs. This occurs if the switching arms 68, 69 are turned clockwise to the position 71, the two arms 68 and 69 being mechanically connected to each other. This position is so selected that shortly before, a current flows through the coils 21, 23 and 25, the current guaranteeing the actuation of the second magnet pairs. Now that the connection between the potentiometer 66 and the voltage source U required for displacement of the part has been established, the displacement movement can becarried out by means of a change of the potentiometer tap.

When the displacement movement has ended, the switch arm 68 is turned so that the part of resistor 70 remaining in the left current circuit always gets smaller and, accordingly, the pressure force between the'displaceable part 5 on the one hand and its support surface 11 on he other hand, always gets correspondingly larger. Finally, in the position 72 the pressure force is sufficiently large and the right switching arm 69 slides off of the extension 73 and opens the right current circuit. By turning the switching arm to the position 74, the left current circuit is also broken and the displaceable part 5 can fall, so that it now rests on the pole faces of magnets 21 and 23 illustrated in FIG. 1.

In an alternate embodiment to that illustrated in FIG. 1, the coils that are fixedly mounted to the particle-beam device can be positioned outside the evacuated space. In addition, the excitation current of all electromagnets can contain a ripple portion. A ripple current with decreasing amptitude can serve, if required, to eliminate remanence. Also, shielding means can be provided.

According to another embodiment of the invention, means are provided for generating electrical signals as actual values that correspond to specific excitations of the electromagnets of the first magnet pairs and, consequently, the electrical signals correspond to specific positions of the displaceable part. And, additional means are provided for storing these signals as reference values formeasuring the above-mentioned excitations. This makes it possible to reproduce definite positions of the displaceable part in a simple manner. This is of special significance if the displaceable partis the specimen table of an electron microscope and it is of interest to study definite locations on the specimen after an overall view of the makeup of the specimen has been obtained.

This embodiment of the invention is always usable in other application situations where it is desired to have a characterizing criteria for the coordinates of a displaceable part in the evacuated space of a particle-beam device. For example, with devices for evaluating materials by means of a charge carrier beam, a plurality of specimens can be advantageously arranged on he specimentable. In this instance, it is of interest to have a criteria as to which specimen is under the working beam at a particular time. A similar requirement occurs with regard to diaphragms adjustable transversely with respect to the beam and with diaphragm rods in which are seated several diaphragms.

It is advantageous to supply only the electrode-magnets on one side of the displaceable part with an excitation dependent upon the desired position of the part, whereas the electromagnets lying on the other side are supplied with a constant excitation. It is then only necessary to generate and store signals which correspond to the dependent excitation. As shown in connection with FIG. 3, it is a simple matter to control the current supply for the excitation of the electromagnets by using a potentiometer so that when an adjustment of the excitation of the electromagnets lying on the one side of the displaceable part is madeso as to increase the excitation, the excitation of the electromagnets lying on the opposite sideis reduced in the same amount. It can be advantageous to use a device that forms the difference from the two excitation current values and which translates this difference to obtain a positioning signal. Since, with such a current supply, the changes in the excitation current of the electromagnets disposed on both sides of the displaceable part are dependent on one another,

the changes being obtained by positioning a potentiometer of the current supply, a single memory or storage unit can serve for measuring both excitation currents. his then sufficient to store only the signals of one side.

The actual value signals can be taken directly from the excitation currents of the electromagnets. The position of the displaceable part can itself be evaluated for obtaining the actual value signal. In this connection, for example, a wedge-like member featuring a lengthwise increasing attenuation can be joined to'the displaceable part and arranged thereon so as to be disposed between a radiation source at a fixed location and a radiation sensitive component of the signal current circuit. These wedge-like members should have an attenuation that increases strongly in one direction with small changes in the position of the displaceable part. In view of the small movements, the radiation sources should be so constructed that they irradiate the wedge-like member with a pencil point beam.

With the arrangements already described, it is preferable to provide throw-over switching so-that it is possible to connect the storage means selectively to the signal conducting circuit. This switching means is then actuated, for example, if the first running of the specimen in the electron microscope discloses interesting specimen details.

In the event that the electrical signals are taken directly from the excitation current or the excitation voltage of the electromagnet, an additional throw-over switch is arranged so that the electromagnet can be selectively connected with a direct-current voltage sources for obtaining their excitation currents or with a storage or memory means. Especially in this case of taking the signals directly from the excitation currents or excitation voltages, a converter would be used which con verts the signals to an advantageous form for storage, for example, the signals could be converted to binary signals. Also, at different locations amplifiers may be provided.

Basically, this other embodiment includes such arrangement wherein the storage unit or memory is similar to an instrument that permits reading the stored values as reference values and wherein the displacements of the displaceable part are made manually until actual values are indicated which correspond to the reference values. However, it is more sophisticated to use a controller having an input to which are supplied the actual and stored reference values for retrieving the position of the displaceable part and. in response to a matching of signals, preclude further movements of the part.

The above-discussed other embodiment of the invention will now be explained with reference to FIGS. 4 and 7 of the drawing. ln each of these drawings, the invention is discussed with reference to an electron microscope wherein a specimen table is displaceable transversely to the beam axis as in FIGS. 1 and 2.

H6. 4 illustrates an. arrangement of the invention for obtaining the movement of a displaceable table along a coordinate axis.

FIG. 5 illustrates the specimen table provided with wedgeshaped members featuring a lengthwise increasing attenuation. v a

FIG. 6 illustrates schematic block diagram of a control circuit functioning in cooperative relation with the wedgeshaped members of FIG. 5. The circuit is applied for automatically obtaining a specific position of a specimen carried by the specimen table.

FIG. 7 illustrates the schematic block diagram of a control circuit with separate regulators for two coordinates, that is for movements which are usually required with a specimen table in particle-beam devices.

FIG. 4 illustrates an arrangement of the invention for positioning an object table in the coordinate .r-direction. Two electromagnet pairs 102 and 103 are disposed at diametrically opposite sides of the table 101. The inner magnets 104 and 105, the two magnet pairs are fixedly mounted to the table 101, whereas the outer magnets 106 and 107 are fixedly mounted to the particle-beam device. Reference numeral 108 indicates the trace of the particle-beam on the specimen table.

The magnet coils are supplied from a current adjuster not illustrated in FIG. 4. The'current circuits shown extending upwardly toward respective coil pairs 102 and 103 are connected to the current adjuster in the direction of the respective arrows of those circuits. It is noted that the current circuits of FIGS. 4, 6 and 7 are depicted by single lines to facilitate representation. Actually, the circuits are closed circuits analogous to those in FIG. 3.

One possibility of obtaining a characterizingcriteria for positioning the table 101 corresponding to especially interesting specimen details lying in'the region of the beam 108 is given by switch 109 by connecting the memory 110 with the supply current circuit of the magnet pair 102, the supply current circuit being shown in the left part of FIG. 4. A converter 111 is connected therebetween which functions to convert the signals given by the intensity of the direct-current i to signals that are suitable for storage in the memory 110. As soon as it is required to secure the position of an interesting specimen position with regard to its x-coordinate, the switch 109 is thrown over and the value of the current i corresponding to the specimen position is stored as a reference character of the abscissa value of this position. lf it is desired to view this specimen position at a later time, the switch 112 is thrown over from its illustrated position so that it connects the magnet pair 102 with the memory 110 via the converter 113. The converter 113 is a current generator that provides a direct current to the magnet pair 102 for exciting the same. This direct current is specific in its magnitude because ofthe reference value stored in the memory 110. During this displacement movement procedure, the switch 109 can be open. Because the switch 112 has interrupted the direct excitation current circuit for the magnet pair 102, the switch 109 can also remain closed. In all instances, a memory 110 is used which can store a larger number of interesting specimen positions with reference to the x-axis. Then the converter 109 can be correspondingly often closed during the search of the specimen for interesting regions. The magnets 104 and 106 and 105, 107 of the respective pairs 102 and 103 are poled so that repelling forces are developed between the magnets of each pair. To develop the force difference required for a displacing movement of table 101, only the excitation of the magnets 104 and 106 is changed, whereas the excitation of magnet pair 103 remains constant. If it is desired to also change the excitation pair of magnet 103, it is necessary to take signals from the current supply circuit of magnets I and 107 and store these signals as reference values.

FIG 5 shows the table 120 having a specimen arranged in the region of the beam 121. For carrying out the displacement of this table in the .r-coordinate direction there are again provided opposite like magnet pairs 122 and 123. Additional magnet pairs 124 and 125 are arranged perpendicular to magnet pairs I22 and 123. Magnet pairs 122 and 123 serving to position for displacement in the y-coordinate direction.

In order to establish reference positions of the table 120 required for examining interesting specimen details, the embodiment according to FIGS. 5 and 6 provide that wedgeshaped members 126 and 127 be arranged on the table in the regions of respective magnet pairs 122 and 124. Both wedgeshaped members feature an attenuation that continuously in creases in the lengthwise direction over the entire member so that with regard to a fixed point, the attenuation ofthe wedgeshaped member is a reference for the corresponding position of the table 120.

The same reference numerals are used in FIG. 6 as are used in FIG. 5. FIG. 6 shows table 120 with magnet pairs 122 and 123 as well as the wedge-shaped member 126. Also shown are additional details of the construction. A ray or beam generator 126 a is shown disposed above the arrangement. Generator 1260 produces visible light which impinges upon a small region of wedge-shaped member 126 in pencil form, the light being concentrated by a lens 128. The light beam passes through the wedge-shaped member 126 which modifies the beam in accordancewith the position of the table 120. After passing through the wedge-shaped member, the modified beam impinges upon a detector 129 that is sensitive to changes made to the beam by the wedge-shaped member 126. The radiation detector 129 is part of a current circuit and in fluences thecurrent in this circuit according to changes in the light beam. Such a detector 129, could be, for example, a photo-resistor.

A radiation detector 129 delivers signals to a signal converter 131 via an amplifier 130. These signals S1 are converted, for example, into binary or other coded signals S2 so as to be suitable for storage in the memory 132. As soon as switch 133 is pressed, the storage action occurs. 7

The embodiment shown in FIGS. 5 and 6 enables, as desired, the attainment, automatically, ofa new position of the table is corresponding to a pertinent specimen location. Regulator 134 serves to achieve this effect and is constructed according to the follower principle of regulation. During the usual scanning of the specimen and during the storage procedure, the switch 135 in the illustrated position. A magnet pair 122 is directly connected with the current adjuster. The automatic retrival ofa pertinent specimen detail is introduced by throwing over switch 135, so that the supply current circuit for the magnetic pair 122 is broken and switched so as to' receive its current via the regulator 134. Regulator 134 has a positioning motor that positions potentiometer 136 in the present supply current circuit for the magnet pair 122 as long as there is a difference between the stored reference valves S2 and the actual values S2 measured by the detector 129. In principle, the arrangement concerns a regulator and is essentially a difference amplifier with the positioning motor. Only when the difference between signals S2 and S2 has vanished, this positioning of the potentiometer I36 and therewith the positioning ofthe table 120 is discontinued.

' FIG. 7 shows the block diagram of a circuit for automatically positioning a specimen table in the x and y coordinate directions for examining pertinent specimen details. Reference numerals 140 and 141 designate respective radiation detectors analogous to the detector 129 in FIG. 6. Detectors and 141 are arranged with respect to the x and y coordinates respectively. Signals are supplied to converters 144 and via amplifiers 142 and 143 respectively. The converters 144 and 145 convert signals S1 and S1,, into signals S2, and S2,, respectively suitable for storage in memory 146. For receiving signals to be stored, the memory 146 can be connected via switch 147 to the supply current circuits for movement in the x and y directions.

In the arrangement according to FIG. 7, separate regulators 148 and 149 are provided for movements in the respective .r and y directions. Specific value signals 52, and S2,, as well as the reference values S2, and S2, from memory 146 are fed into the regulators 148 and 149 in a manner analogous to that of FIG. 6. The regulators modify the excitation currents of the magnet pairs arranged to impart displacing movements to the table in the x and y directions. The switches I50 and 151 correspond to switch 135 of the arrangement according to FIG. 6.

To those skilled in the art it will be obvious upon a study of this disclosure that our invention permits of a great variety of modifications and hence may be given embodiments other than those illustrated and described herein without departing from the essential features of our invention and the scope of the claims annexed hereto.

We claim:

1. In a particle-beam device having a beam axis and a housing surrounding said axis, a moveable structure within said housing and an apparatus for moving said structure transverse to said beam axis, said apparatus comprising first magnet pairs disposed at opposite sides of said structure and positioned so that their respective magnetic forces act in a direction transverse to said axis, the two magnets of each pair of said first magnet pairs being mutually adjacent, one of said adjacent magnets being arranged on the outer periphery of said structure, the other one of said two adjacent magnets being fixedly mounted with respect to said housing, second magnet pairs positioned relative to said axis so that their respective magnetic forces act in a direction parallel to said axis, the magnets of each pair of said second magnet pairs being mutually adjacent, one magnet of each pair of said second magnet pairs being arranged on a surface of said structure, said surface being transverse to said axis, the other magnet of each pair of said second magnet-pairs being fixedly mounted with respect to said housing, supporting means having a load surface adjacent said structure and transverse to saidaxis, at least one magnet of each pair of said first magnet pairs and said second magnet pairs being an electromagnet, and electric control means connected to the electromagnets of said first magnet pairs for energizing the same, for applying a force to said structure to move the same transverse to said beam axis, said control means also being connected to the electromagnets of said second magnet pairs for energizing the same for applying a force to said structure to hold said structure against said surface creating a friction force therebetween while said structure is moved transversely.

2. An apparatus according to claim 1 wherein said control means includes adjusting means for making a first adjustment of the excitation of said electromagnets of said second magnet pairs for holding said structure against said surface creating a friction force therebetween greater than said previously mentioned friction force while removing the excitation of "said electromagnets of said first magnet pairs.

3. An apparatus according to claim 2, wherein said load surface is disposed above said structure, and said apparatus comprises second supporting means having a support surface transverse to said beam axis, said structure being disposed with clearance intermediate said load surface and said support surface, the latter being disposed beneath said structure, said adjusting means including circuit means for making a second adjustment of the excitation of 'said electromagnets of said second magnet pairs after said removal of the excitation of said electromagnets of said first magnet pairs, whereby said structure drops to rest on said support surface of said second supporting means.

4. An apparatus according to claim 3, said other magnets of said second magnet pairs having respective pole pieces, the latter having respective pole faces in a common plane, said support surface being said respective pole faces.

5. An apparatus according to claim 4, said one magnets of said second magnet pairs having respective pole pieces adjacent corresponding ones of said pole pieces of said other magnets of said second magnet pairs, said pole pieces of said one magnets of said second magnet pairs having respective pole faces resting on corresponding ones of said pole faces of said pole pieces of said other magnets of said second magnet pairs in response to said second adjustment of the excitation of said electromagnets of said second magnet pairs.

6. An apparatus according to claim 3, said circuit means including means for reversing the polarity of each magnet pair of said second magnet pairs;

7. An apparatus according to claim 2, said second magnet pairs being three in number, the electromagnets of said second magnet pairs being wound so as to have substantially identical ampere turns.

8. An apparatus according to claim 2, said first magnet pairs including first and second magnet pairs respectively disposed at diametrically opposite sides of said structure, said electric control means including ancillary means for adjusting the polarity and excitation of the respective electromagnets of said last-mentioned first and second magnet pairs so that, for a direction of movement of the structure toward the magnet of said second magnet pair communicating with said housing so as to be fixedly mounted with respect thereto, the adjacent magnets of said second magnet pair exert a repelling force on each other smaller than the repelling force exerted by the adjacent magnets of said first magnet pair.

9. An apparatus according to claim 8, said ancillary means including a potentiometer common to the respective electromagnets of said last-mentioned first and second magnet pairs for simultaneously adjusting the respective excitations of said respective electromagnets.

10. An apparatus according to claim 2, said electric control means comprising respective excitation circuits connected to the electromagnets of said second magnet pairs and the electromagnets of said first magnet pairs, and a sequence switch connected into said excitation circuits for first closing the excitation circuit of said second magnet pairs and then closing the excitation circuit of said first magnet pairs. I

11, An apparatus according to claim 1 comprising third magnet pairs disposed at opposite sides of said structure and positioned so that their respective magnetic forces act in a direction to prevent undesired rotations of said structure, the magnets of each pair of said third magnet pairs being mutually adjacent, one magnet of each pair of said third magnet pairs being arranged on the outer periphery of said structure, the other of each pair of said third magnet pairs communicating with said housing so as to be fixedly mounted with respect thereto.

12. An apparatus according to claim 11, said third magnet pairs being positioned 45 from said first magnet pairs.

13. An apparatus according to claim 11. the magnets of each pair of said third magnet pairs being poled so as to apply pulling forces on said structures.

14. An apparatus according to claim 13, the pulling forces exerted by said third magnet pairs on said structure being greater in magnitude than the repelling exerted by said first magnet pairs on said structure.

15. An apparatus according to claim 1, said structure being a displaceable specimen table.

16. An apparatus to claim 1, at least one magnet of each pair of said first magnet pairs being an electromagnet, said apparatus comprising control means connected to said elec-.

tromagnets for generating respective electrical signals that correspond to specific excitation values of said electromagnets and thereby correspond to specific positions of said structure and memory means connected to said control means for storing said signals as reference for said excitation values.

17. An apparatus according to claim 16, said first magnet pairs comprising two magnet pairs disposed at respectively opposite sides of said structure, said control means comprising ancillary means connected between said memory means and said one electromagnet of one pair of said two magnet pairs for supplying said one electromagnet of said one pair with a signal excitation dependent upon the desired position of the structure, said signal excitation corresponding to signal values stored in said memory means, and means for supplying a constant excitation to said one electromagnet of the other pair of said two magnet pairs.

18. An apparatus according to claim 16, said control means comprising excitation circuits connected to said one magnets respectively, ray generator means fixedly mounted with respect to said housing, ray detector means connected to at least one of said excitation circuits, and wedge-shaped members disposed on said structure so as to be intermediate said ray generator means and'said ray detector means, said members having an attenuation increasing continuously in a single direction.

19, An apparatus according to claim 16, said control means comprising excitation circuits connected to said one magnets respectively, memory means for storing signals, and switching means for connecting and disconnecting said memory means with at least one of said excitation circuits,

20. An apparatus according to claim 19, said control means comprising direct-current supply means, and ancillary switching means forselectively connecting said one magnets with said memory means and said supply means.

21. An apparatus according to claim 16, said control means comprising subsidiary means for providing specific value signals corresponding to specific positions of said structure, said memory means providing reference value signals, regulator means connected to said subsidiary means and said memory means for comparing said reference value signals and said specific value signals, and positioning means connected to said regulator and engaging for positioning said structure until said specific value signals are the same as said reference value signals. 

1. In a particle-beam device having a beam axis and a housing surrounding said axis, a moveable structure within said housing and an apparatus for moving said structure transverse to said beam axis, said apparatus comprising first magnet pairs disposed at opposite sides of said structure and positioned so that their respective magnetic forces act in a direction transverse to said axis, the two magnets of each pair of said first magnet pairs being mutually adjacent, one of said adjacent magnets being arranged on the outer periphery of said structure, the other one of said two adjacent magnets being fixedly mounted with respect to said housing, second magnet pairs positioned relative to said axis so that their respective magnetic forces act in a direction parallel to said axis, the magnets of each pair of said second magnet pairs being mutually adjacent, one magnet of each pair of said second magnet pairs being arranged on a surface of said structure, said surface being transverse to said axis, the other magnet of each pair of said second magnet pairs being fixedly mounted with respect to said housing, supporting means having a load surface adjacent said structure and transverse to said axis, at least one magnet of each pair of said first magnet pairs and said second magnet pairs being an electromagnet, and electric control means connected to the electromagnets of said first magnet pairs for energizing the same for applying a force to said structure to move the same transverse to said beam axis, said control means also being connected to the electromagnets of said second magnet pairs for energizing the same for applying a force to said structure to hold said structure against said surface creating a friction force therebetween while said structure is moved transversely.
 2. An appAratus according to claim 1 wherein said control means includes adjusting means for making a first adjustment of the excitation of said electromagnets of said second magnet pairs for holding said structure against said surface creating a friction force therebetween greater than said previously mentioned friction force while removing the excitation of said electromagnets of said first magnet pairs.
 3. An apparatus according to claim 2, wherein said load surface is disposed above said structure, and said apparatus comprises second supporting means having a support surface transverse to said beam axis, said structure being disposed with clearance intermediate said load surface and said support surface, the latter being disposed beneath said structure, said adjusting means including circuit means for making a second adjustment of the excitation of said electromagnets of said second magnet pairs after said removal of the excitation of said electromagnets of said first magnet pairs, whereby said structure drops to rest on said support surface of said second supporting means.
 4. An apparatus according to claim 3, said other magnets of said second magnet pairs having respective pole pieces, the latter having respective pole faces in a common plane, said support surface being said respective pole faces.
 5. An apparatus according to claim 4, said one magnets of said second magnet pairs having respective pole pieces adjacent corresponding ones of said pole pieces of said other magnets of said second magnet pairs, said pole pieces of said one magnets of said second magnet pairs having respective pole faces resting on corresponding ones of said pole faces of said pole pieces of said other magnets of said second magnet pairs in response to said second adjustment of the excitation of said electromagnets of said second magnet pairs.
 6. An apparatus according to claim 3, said circuit means including means for reversing the polarity of each magnet pair of said second magnet pairs.
 7. An apparatus according to claim 2, said second magnet pairs being three in number, the electromagnets of said second magnet pairs being wound so as to have substantially identical ampere turns.
 8. An apparatus according to claim 2, said first magnet pairs including first and second magnet pairs respectively disposed at diametrically opposite sides of said structure, said electric control means including ancillary means for adjusting the polarity and excitation of the respective electromagnets of said last-mentioned first and second magnet pairs so that, for a direction of movement of the structure toward the magnet of said second magnet pair communicating with said housing so as to be fixedly mounted with respect thereto, the adjacent magnets of said second magnet pair exert a repelling force on each other smaller than the repelling force exerted by the adjacent magnets of said first magnet pair.
 9. An apparatus according to claim 8, said ancillary means including a potentiometer common to the respective electromagnets of said last-mentioned first and second magnet pairs for simultaneously adjusting the respective excitations of said respective electromagnets.
 10. An apparatus according to claim 2, said electric control means comprising respective excitation circuits connected to the electromagnets of said second magnet pairs and the electromagnets of said first magnet pairs, and a sequence switch connected into said excitation circuits for first closing the excitation circuit of said second magnet pairs and then closing the excitation circuit of said first magnet pairs.
 11. An apparatus according to claim 1 comprising third magnet pairs disposed at opposite sides of said structure and positioned so that their respective magnetic forces act in a direction to prevent undesired rotations of said structure, the magnets of each pair of said third magnet pairs being mutually adjacent, one magnet of each pair of said third magnet pairs being arranged on the outer periphery of said structure, The other of each pair of said third magnet pairs communicating with said housing so as to be fixedly mounted with respect thereto.
 12. An apparatus according to claim 11, said third magnet pairs being positioned 45* from said first magnet pairs.
 13. An apparatus according to claim 11, the magnets of each pair of said third magnet pairs being poled so as to apply pulling forces on said structures.
 14. An apparatus according to claim 13, the pulling forces exerted by said third magnet pairs on said structure being greater in magnitude than the repelling exerted by said first magnet pairs on said structure.
 15. An apparatus according to claim 1, said structure being a displaceable specimen table.
 16. An apparatus to claim 1, at least one magnet of each pair of said first magnet pairs being an electromagnet, said apparatus comprising control means connected to said electromagnets for generating respective electrical signals that correspond to specific excitation values of said electromagnets and thereby correspond to specific positions of said structure and memory means connected to said control means for storing said signals as reference for said excitation values.
 17. An apparatus according to claim 16, said first magnet pairs comprising two magnet pairs disposed at respectively opposite sides of said structure, said control means comprising ancillary means connected between said memory means and said one electromagnet of one pair of said two magnet pairs for supplying said one electromagnet of said one pair with a signal excitation dependent upon the desired position of the structure, said signal excitation corresponding to signal values stored in said memory means, and means for supplying a constant excitation to said one electromagnet of the other pair of said two magnet pairs.
 18. An apparatus according to claim 16, said control means comprising excitation circuits connected to said one magnets respectively, ray generator means fixedly mounted with respect to said housing, ray detector means connected to at least one of said excitation circuits, and wedge-shaped members disposed on said structure so as to be intermediate said ray generator means and said ray detector means, said members having an attenuation increasing continuously in a single direction. 19, An apparatus according to claim 16, said control means comprising excitation circuits connected to said one magnets respectively, memory means for storing signals, and switching means for connecting and disconnecting said memory means with at least one of said excitation circuits.
 20. An apparatus according to claim 19, said control means comprising direct-current supply means, and ancillary switching means for selectively connecting said one magnets with said memory means and said supply means.
 21. An apparatus according to claim 16, said control means comprising subsidiary means for providing specific value signals corresponding to specific positions of said structure, said memory means providing reference value signals, regulator means connected to said subsidiary means and said memory means for comparing said reference value signals and said specific value signals, and positioning means connected to said regulator and engaging for positioning said structure until said specific value signals are the same as said reference value signals. 